services: nfs: Add nfs-service-type.
* gnu/services/nfs.scm (<nfs-configuration>): New record.
(nfs-configuration, nfs-configuration?, nfs-configuration-nfs-utils,
nfs-configuration-nfs-version, nfs-configuration-exports,
nfs-configuration-rpcmountd-port, nfs-configuration-rpcstatd-port,
nfs-configuration-rpcbind, nfs-configuration-idmap-domain,
nfs-configuration-nfsd-port, nfs-configuration-nfsd-threads,
nfs-configuration-pipefs-directory, nfs-configuration-debug,
nfs-shepherd-services): New procedures.
(nfs-service-type): New variable.
* doc/guix.texi (Network File System): Document it.
* gnu/tests/nfs.scm (%test-nfs-server): New variable.
(%base-os): Use default value of rpcbind-service-type.
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